Fundamental problems in the chemical process industry include management of reaction equilibrium and kinetics to achieve high conversion with desired selectivity under moderate reaction conditions, and management of the heat of reaction to control reaction temperature and to achieve high energy efficiency.
Typically, single pass conversion of the feed reactant(s) is incomplete because of equilibrium limitations. It is then often necessary to provide a separation system to extract useful products from the reactor effluent, and then to recycle unconsumed reactants to the reactor inlet. The prior art provides known separation processes based on condensation, distillation, membrane permeation, absorption, and adsorption. In most cases, these prior art separation processes are incompatible with the operating temperature of the reaction itself. Most conventional separation processes operate at ambient or sub-ambient temperature, while the reaction operates at elevated temperature so that costly heat exchangers are required for the recycle loop.
High temperatures generally promote good reaction rates, but shift the equilibrium of exothermic reactions toward lower conversion. The high cost of heat exchangers, recycle compressors, and other auxiliary equipment then incentivizes operation at relatively severe pressure or temperature reaction conditions to minimize the need for recycle. In the typical exothermic reaction example of ammonia synthesis, satisfactory conversion is achieved by forcing the equilibrium with high pressure operation, while product separation from the recycle loop is achieved by condensation usually after refrigeration.
Important applications exist where the separation of carbon dioxide is desired at elevated temperature from a reactive gas mixture containing steam, or where such separation could greatly enhance process efficiency, simplicity and economics. An important example is hydrogen production by steam reforming of natural gas. Several prior art processes have proposed sorption to remove carbon dioxide from reacting mixtures of steam and methane in order to drive the steam reforming and water gas shift reaction equilibria in order to produce moderately pure hydrogen at high conversion. Use of lime as a thermally regenerated sorbent in a fluidized bed reactor was proposed by Brun-Tsekhovoi et al, “The Process of Catalytic Steam-Reforming of Hydrocarbons in the Presence of Carbon Dioxide Acceptor”, Hydrogen Energy Progress VII, Proceedings of the World Hydrogen Energy Conference, Pergamon Press, p. 885 (1988). More recently, fixed bed pressure swing adsorption reactor processes for steam methane reforming have been developed by Gaffney et al (U.S. Pat. No. 5,917,136) using modified alumina adsorbents, and by J. R. Hufton, S. G. Mayorga and S. Sircar (“Sorption Enhanced Reaction Process for Hydrogen Production”, AIChEJ 45, 248 (1999)) using mixed metal oxides derived from hydrotalcite and promoted with potasssium carbonate.
Gas separation by pressure swing adsorption (PSA) is achieved by coordinated pressure cycling and flow reversals over an adsorbent bed which preferentially adsorbs a more readily adsorbed component relative to a less readily adsorbed component of the mixture. The total pressure is elevated during intervals of flow in a first direction through the adsorbent bed from a first end to a second end of the bed, and is reduced during intervals of flow in the reverse direction. As the cycle is repeated, the less readily adsorbed component is concentrated in the first direction, while the more readily adsorbed component is concentrated in the reverse direction.
A “light” product, depleted in the more readily adsorbed component and enriched in the less readily adsorbed component, is then delivered from the second end of the bed. A “heavy” product enriched in the more strongly adsorbed component is exhausted from the first end of the bed. Typically, the feed is admitted to the first end of a bed and the second product delivered from the second end of the bed when the pressure in that bed is elevated to a higher working pressure, while the second product is exhausted from the first end of the bed at a lower working pressure which is the low pressure of the cycle.
The conventional process for gas separation by pressure swing adsorption uses two or more adsorbent beds in parallel, with directional valving at each end of each adsorbent bed to connect the beds in alternating sequence to pressure sources and sinks, thus establishing the changes of working pressure and flow direction. This conventional pressure swing adsorption process makes inefficient use of applied energy, because of irreversible expansion over the valves over large pressure differences while switching the adsorbent beds between higher and lower pressures.